This invention relates to a power transmission unit primarily intended for motor vehicle use, and more particularly to a transmission of this type which will function to automatically provide a plurality of different forward drive speeds and also a reverse drive, the transmission unit including a torque converter, the elements of which may be mechanically locked together to reduce slippage and increase efficiency.
In automatic transmission for motor vehicles, a torque converter is usually provided to insure a power flow from a stopped condition to a cruising condition which is smooth and shock-free. Using transmission fluid as its only medium of torque transmission, the torque converter slips when the car is at rest with the engine idling, thus eliminating the need for a clutch. For acceleration, its impeller, turbine and stator multiply engine torque in infinitely variable ratios up to a maximum of such as 2.1 to 1 for example. Thus, it is impossible to stall the engine even on the steepest grade. The torque converter merely slips in such a situation.
As the vehicle accelerates from a stopped condition to a cruising condition the torque converter slippage ranges from complete or full down to a minimum or very little amount, such as five (5) percent, so the torque converter achieves a substantially locked up condition (except for such low percentage slip). In such condition there is no torque multiplication and very little slippage. However, prior to reaching a predetermined cruising speed the torque converter does slip. To inhibit slippage, both at low speeds as well as high speeds, various clutch devices have been employed in the past. For example, U.S. Pat. No. 2,667,085 shows a clutch adapted to drivingly connect the impeller and turbine of a torque converter together. This invention is directed to a clutch for locking the torque converter elements together at speeds above a predetermined minimum.